Leptospirosis is a common zoonotic disease, typically transmitted to humans by rat exposure. Leptospirosis is emerging in areas of the world undergoing rapid urbanization and was recently designated by the National Institutes of Health as an Emerging Infectious Disease. Research is urgently needed to understand how these pathogenic spirochetes interact with their mammalian hosts. The focus of this proposal is to elucidate the molecular mechanisms of pathogenesis and immunity mediated by outer membrane proteins (OMPs). Our laboratory, a leader in this field, has developed strategies for membrane fractionation and has described genes encoding six leptospiral OMPs. We have shown that two of these OMPs, a porin (OmpL1) and a surface-exposed lipoprotein (LipL41) are synergistically immunoprotective in the hamster model of leptospirosis. Recently, we have begun to study the function of leptospiral OMPs using genetic manipulation of leptospires. In the Progress Report we announce the discovery of a novel class of leptospiral proteins containing Bacterial ImmunoGIobulin-like (Big) repeats. The leptospiral Big proteins are structurally related to the Intimin/Invasin family of bacterial adhesins. One of these leptospiral Big proteins, designated BigL3, confers a cellular adhesin phenotype on E. coll. Specific Aim 1 describes studies designed to localize BigL3 and other novel candidate OMPs to be identified through emerging genomic sequence and proteomics data. The goal of Specific Aim 2 is to examine the role of OMPs in mechanisms of pathogenesis and immunity using in vitro assays of adhesion, bactericidal activity, growth inhibition, and opsonophagocytosis. Specific Aim 3 will determine whether any of the OMPs found to play a role in mechanisms of pathogenesis and immunity in Specific Aim 2 are immunoprotective in mucosal, subcutaneous, and intraperitoneal challenge models of hamster leptospirosis. In Specific Aim 4, epitope mapping and monoclonal antibody studies will be performed with the goal of identifying biologically relevant OMP epitopes. The information gained from these studies will be essential for development of rational approaches for the diagnosis, prevention, and treatment of leptospirosis.